Bragg gratings can be created in a .suitably photoresponsive optical waveguide by illuminating it with an interference fringe pattern of appropriate pitch generated in ultra-violet light of a suitable wavelength, typically in the region of about 200 nm. A first order Bragg grating in such a waveguide may typically have a pitch of about 1.0 .mu.m.
One technique by which such an interference fringe pattern can be generated is to arrange for two coherent beams of collimated ultra-violet light to intersect each other at an appropriate angle. The wavefronts of these two beams are respectively schematically represented in FIG. 1 by the sets of parallel lines 10 and 11. Their interference sets up a fringe pattern in which an optical fibre 12 is placed. The resolved component of the pitch of the fringes, measured along the axis of the fibre, depends upon the wavelength of the light being employed, the angle at which the two beams intersect, and the angle between the fibre axis and the resulting fringe pattern.
The spectral selectivity of a Bragg grating depends in part upon the length of that grating. The production of long interference patterns in the manner described with reference to FIG. 1 requires the use of correspondingly wide beams, which in its turn imposes increased constraints upon the coherence length of the light emitted by the source employed to generate these beams, and upon the optical quality of the associated optical system. Alternatively, narrower beams of shorter coherence length could be employed together with some mechanical `step-and-repeat` arrangement to step the position of the interference pattern with respect to the waveguide in which the Bragg grating is being created. However this alternative approach is seen to suffer from the disadvantage of requiring mechanical translation stages of &lt;100 nm absolute precision over the required length of the Bragg grating.
A different approach to the generation of the interference fringes is to employ a diffraction grating, and to locate the waveguide in close proximity to its diffracting elements. This arrangement is for instance described in a paper by D M Tennant et al. entitled; `Characterization of near-field holography grating masks for optoelectronics fabricated by electron beam lithography`, J Vac. Sci. Technol. B, Vol. 10, No. 6, pp 2530-5 (Dec 1992). That paper is particularly concerned with using an e-beam written phase grating etched into fused silica substrate as the diffraction grating, and the design of the phase mask was such that, when it was illuminated with ultra-violet light at 364 nm at the appropriate angle of incidence, only the zero order and first order diffracted beams propagated in the region of space beyond the phase mask. The (near-field) interference between these two beams was then used to write a photolithographic mask on an InP substrate for subsequent etching to produce the DFB grating structure of a DFB laser. When this general approach is attempted for the direct writing of Bragg gratings in photosensitive glass waveguides, a crucial difference is encountered that is attributable to the much lower refractive index of glass than InP. This means that the required pitch of the Bragg grating in the glass is much greater than in the InP, typically 1.08 .mu.m as opposed to 0.24 .mu.m. In the case of the writing of the fringe pattern in a photoresist layer on InP, the wavelength of the writing beam, at 364 nm, was longer than the pitch of the fringe pattern, which lay in the range 235 to 250 nm, and hence the non-evanescent propagation of a second order diffracted beam beyond the phase grating does not occur. However in the case of the writing of a fringe pattern having a pitch of about 1 .mu.m using a writing beam with a wavelength of about 240 nm, non-evanescent propagation of at least second and third order diffracted beams beyond the phase grating is not automatically suppressed, but is very liable to occur unless the profile of the diffracting elements is specifically designed to suppress any of these higher order diffracted beams.
FIG. 2 illustrates the situation in respect of a short phase grating of pitch 1.08 .mu.m, 1:1 mark space ratio and phase .pi., that is illuminated at normal incidence with light of 244 nm wavelength. The phase step .pi. suppresses the zero order beam, but the first, second, and third order beams are propagated. The near field intensity is plotted in 5 .mu.m steps from 5 .mu.m out to 300 .mu.m from the phase grating. It is seen that the near field interference is no longer the simple fringes resulting from two-beam interference, but has a more complicated structure.
For FIG. 2 it was chosen to exemplify the near field intensity pattern in respect of a short grating only 40 periods long so that the separation of the different diffraction orders is more readily apparent. Normally the grating would be much longer than this, with the result that the different orders remain substantially totally overlapped out to a much greater distance from the grating. This is the situation exemplified in the near field intensity patterns plotted in FIG. 3. The illumination conditions for FIG. 3 are the same as those for FIG. 2, and the sole difference between the gratings is that the FIG. 3 grating is 1000 periods long instead of only 40. In FIG. 3 the intensity profile is plotted in 2 .mu.m steps from 40 .mu.m out to 200 .mu.m from the grating. FIG. 3 shows that if the waveguide core were spaced from the grating at an optical path distance equivalent to 70 .mu.m in air, then the resulting photo-induced Bragg grating would have the same principal periodicity as that of the phase grating, whereas if the spacing were at an optical path distance equivalent to 95 .mu.m in air, then the resulting photo-induced Bragg grating would have its principal periodicity equal to half that of the phase grating.
The situation is even more confused if imperfections in the phase grating are taken into account. FIG. 4 is a replot of the intensity profiles of FIG. 3 with the sole difference that the FIG. 4 phase grating has a 10% error in mask height, thus corresponding to a phase step of 0.9.pi. instead of .pi.. Similarly FIG. 5 is a replot of the intensity profiles of FIG. 3 with the sole difference that the FIG. 5 phase grating has a mark space ratio error, being 9:7 instead of 1:1. It is seen that in both instances the errors give rise to intensity variations of high spatial frequency in the near field in the direction normal to the plane of the phase mask.